1. Field of the Invention
This invention is concerned with processes that employ crystalline molecular sieve zeolites as catalysts. It is particularly concerned with processes that use a fixed bed of catalyst to convert an appropriate feed to desired products, and with pretreatment of the feed to make it more readily converted by the catalyst. This invention is advantageous for the catalytic dewaxing of petroleum fuels and lubricants.
2. Prior Art
Modern petroleum refining is heavily dependent on catalytic processes which chemically change the naturally occurring constituents of petroleum. Such processes include hydrocracking, catalytic cracking, reforming and hydrotreating. Historically, the processes all depended on the discovery that chemical change could be induced by contacting a suitable petroleum fraction with a suitable porous inorganic solid at elevated temperature. If hydrogen under pressure is essential to the desired conversion, such as in hydrocracking, a hydrogenation metal is included with the porous catalyst to make the hydrogen effective.
The porous inorganic solids that were originally found useful for catalytic processes included certain clays, aluminas, silica-aluminas and other silicas coprecipitated with magnesia, for example, and such solids are still extensively used in the industry. In general, all of these solids had pores that were not of uniform size, and most of the pore volume was in pores having diameters larger than about 30 Angstroms, with some of the pores as large or larger than 100 Angstroms. As will become evident from the paragraphs which follow, a large fraction of the molecules present in a hydrocarbon feed, such as a gas oil, is capable of entering the pores of the typical porous solids described above.
In recent years much attention has been given to the synthesis and properties of a class of porous solids known as "molecular sieves." These are porous crystalline solids usually composed of silica and alumina, and, because the pore structure is defined by the crystal lattice, the pores of any particular molecular sieve have a uniquely determined, uniform pore diameter. The pores of these crystals are further distinguished from those in the earlier used solids by being smaller, i.e., by having effective pore diameters not greater than about 13 Angstroms. These solids, when dehydrated, act as sorbents that discriminate among molecules of different shape, and for that reason were first called "molecular sieves" by J. W. McBain. The term "effective pore diameter" as used herein means the diameter of the most constricted part of the channels of the dehydrated crystal as estimated from the diameter of the largest molecule that the crystal is capable of sorbing. Zeolite molecular sieves are available that have effective pore diameters ranging from about 3 Angstroms, which is too small to allow occlusion of any hydrocarbon in the pores, to about 13 Angstroms, which allows occlusion of molecules as large as 1,3,5-triethylbenzene. The structures and uses of these solids are described in "Zeolite Molecular Sieves," by Donald W. Breck, John Wiley and Sons, New York (1974), the entire content of which is incorporated herein by reference for background purposes. As indicated by Breck, the zeolite molecular sieves are useful as adsorbents (ibid, page 3), and in catalysts (ibid, page 2).
In spite of the small pores which are characteristic of zeolite molecular sieves, certain of these materials have been found to be highly effective as hydrocarbon conversion catalysts. The conversion of gas oil to gasoline and distillate by catalytic cracking, the alkylation of benzene to ethylbenzene, the isomerization of xylenes and the disproportionation of toluene all involve molecules which are smaller in critical diameter than 1,3,5-triethylbenzene, and such molecules are occluded and acted upon by zeolite molecular sieves having an effective pore diameter of about 10 Angstroms. A particularly interesting catalytic transformation which requires a molecular sieve catalyst is the reduction of the pour point of waxy distillates and residual hydocarbon fractions. Effective pour point reduction depends on the selective convesion of normal, high melting point paraffin molecules that have an effective critical diameter of about 5 Angstroms to substances of lower molecular weight that are easily separated from the low-pour product. Effective catalytic dewaxing depends at least in part on the regularity of the pore size of the crystalline zeolites, which allows selective conversion of unwanted constituents.
The developments briefly described above are only indicative of the commercial importance of the molecular sieve zeolites and of the academic interest in these materials, which is more accurately reflected by the thousands of patents and publications on the subject. By far the major part of this importance stems from the catalytic properties that may be found in appropriate circumstances within the relatively small pores, together with the regularity in the shape of the pores which permits the molecular sieve catalyst to act selectively on molecules having a particular shape. This latter phenomenon has come to be known as "shape-selective catalysis." A review of the state of the catalytic art is found in "Zeolite Chemistry and Catalysis" by Jule A. Rabo, ACS Monograph 171, American Chemical Society, Washington, D.C. (1976), the entire content of which is herein incorporated by reference for background purposes. See particularly Chapter 12 titled "Shape Selective Catalysis."
The dewaxing of oils by shape selective cracking and hydrocracking over zeolites of the ZSM-5 type is discussed and claimed in U.S. Pat. No. Re. 28,398 to Chen et al. U.S. Pat. No. 3,956,102 discloses a particular method for dewaxing a petroleum distillate with a ZSM-5 type catalyst. Typical aging curves are shown in sheet 2 of the drawing of the U.S. Pat. No. 3,956,102. U.S. Pat. No. 3,894,938 to Gorring et al discloses that the cycle life of a ZSM-5 dewaxing catalyst is longer with a virgin feed stream than it is with the same feedstream after it has been hydrotreated. Catalytic dewaxing of petroleum stocks in which a mordenite type of molecular sieve catalyst is used is described in the Oil and Gas Journal, Jan. 6, 1975 issue at pages 69-73. See also U.S. Pat. No. 3,668,113. All of the foregoing patents and the literature reference are herein incorporated by reference.